Amplifier circuit



J. G. PRENTISS AMPLIFIER CIRCUIT.

Filed Oct. 4, 1943 mvzmoa JOHN G. PRENTISS BY M ms; ATTORNEY PatentedJuly 29, T947 AMPLIFIER CIRCUIT John G. Prentiss, Berwyn, 111., assignorto Zenith Radio Corporation, a corporation of Illinois ApplicationOctober 4, 1943, Serial No. 504,958

2 Claims.

This invention relates to a high gain amplifier, and particularly tosuch an amplifier as may be found useful in small radio or hearing aidapparatus. It is desirable to make radio and hearing aid apparatus ofsmall size and with the least number of parts for purposes ofconvenience in carrying and economy in manufacture. In such apparatus,input signals must usually be amplified by a relatively fixed amount foraccomplishing the purposes of the apparatus. Also, it is usuallydesirable to make the amplification of the apparatus relativelyindependent of the magnitude of input signals. The desirable qualitiesof smallness are better realized when the amplification of each stage ishigh and when the number of stages is a minimum.

It is therefore an object of this invention to provide an improved smallhigh gain amplifier stage and particularly one which produces linearamplification.

In the use of hearing aid apparatus, the user must bear a relativelygreat expense for replenishing the power supply. As a general rule, themore electron discharge devices used, in the hearing aid circuit thegreater is the drain on the power supply. Also, it is desirable toincorporate a tone control in the hearing aid apparatus and this isusually accomplished by sacrificing some of the overall amplification orgain realized in the hearing aid circuit; in general, a sacrifice ofoverall gain or amplification is reflected as an increased drain on thepower supply.

Therefore, another object of this invention is to provide an improvedamplifier circuit having a tone control therein and requiring a minimumnumber of electron discharge devices.

Another object of this invention is to provide an improved high gaincircuit requiring a small amount of space current.

The features of my invention which I believe to be novel are set forthwith particularity in the appended claims. My invention itself, both asto its organization and manner of operation, together with furtherobjects and advantages thereof, may best be understood by reference tothe following description taken in connection with the accompanyingdrawing in Which Figure 1 shows an improved hearing aid circuitincorporating a high gain amplifier circuit which embodies my invention;and

Figs. 2, 3 and 4 show a portion of the apparatus of Fig. 1 in variousoperating positions.

In Fig. 1 there is illustrated an improved hearing aid circuit,incorporating a high gain amplifier including electron discharge deviceIt] and a second amplifier including electron discharge device M. Thetwo amplifiers successively amplify signals from microphone II andimpress such amplified signals on sound reproducing device E2 inlinearly amplified form. A switch l3 for effecting tone control isinterconnected with the device [0.

Sound waves impinging on microphone H are transformed into electricalvariations in the main control electrode circuit of discharge deviceIll. The microphone ll may, for example, be of the piezo-electric typeas illustrated, or, because my amplifier produces such great gain, it,may be of the magnetic type. The device Ill greatly amplifies electricalvariations produced by microphone H and the amplified electricalvariations are further amplified by electron discharge device M beforebeing applied to the sound reproducing device l2. One of the importantfeatures of this invention resides in the fact that sound wavesimpinging on microphone I l are reproduced by sound reproducing deviceH2 in greatly amplified form and the degree of amplification issubstantially independent of the magnitude of sound waves impinging onmicrophone I l or signal voltage applied to the main control grid H ofdevice l0.

Microphone I I, which produces electrical variations in response tosound Waves impinging thereon, is connected between the main controlgrid I5 and the grounded filamentary cathode of discharge device it);and a grid leak resistance 16 is connected in parallel circuitrelationship with microphone l l to bypass continuous current flowingaround microphone l I between control grid l5 and the cathode ofdischarge device Iii. Substantially no grid current flows throughresistance l6, because the grid I5 is at a negative potential withrespect to the cathode of device Ill, such negative bias potential beingprovided by connecting grid l5 through resistance 16 to the groundednegative terminal of source 4i] across which are connected the oppositeterminals of the filamentary cathode of device It]. It is to beunderstood, of course, that other conventional means may be used toestablish a suitable continuous operating potential between grid I5 andits cathode and such means may be a battery or a voltage drop producedby space current flowing through discharge device l0. Resistance l'l,much smaller than resistance 1 6, is also connected in parallel tomicrophone ll through switch 13 for a different purpose than resistanceIt as is described hereinafter.

Electron discharge device It) is of the pentagrid 3 type in commercialuse and may, for example, is of the type commonly known as the 1R5. Theparticular elements of this device ill, however, are connected in alinear high gain amplifier circuit in a novel manner not heretoforeknown This invention is not limited to the use of a discharge device ofthe type commonly known as the IRS, but within the scope of my inventionany discharge device which performs equivalent functions is suitable.Large gain is realized when device H3 is connected in the mannerhereinafter described and such large gain is substantially independentof signals applied between the main control grid l5 and cathode ofdischarge device 10.

In general, device I is connected so as to be effectively two amplifiersin cascade with regeneration between the two amplifiers. In addition tothe main control grid l5, discharge device H) has what is termed asecond control grid IS, a suppressor grid [9 connected to the oathode,'amain anode 2ll, and a pair of screen electrodes 2| and 22 on oppositesides of the second control grid l8.

Operatin continuous potentials for device la; are supplied from avoltage source 23 whose negative terminal is grounded and whose positiveterminal is connected to the main anode 23 of discharge device [0through a series circuit including adjustable voltage droppingresistance 24 and output coupling resistance 25. Electrodes 2! and 22are connected together and are maintained positive with respect to thecathode of device Ill by connection to the positive terminal of voltagesource 23 through the series circuit including voltage droppingresistance 24 and coupling resistance 26. The continuous operatingpotential of the second control grid H3 is stabilized by connecting itto ground and the cathode of discharge device If! through resistance 21,which also serves. as a coupling resistance for audio frequency currentas described hereinafter;

.When alternating current signals are impressed between the main controlgrid l and cathode of discharge device l0, substantially all of thealternating output voltage appears across the out-putcoupling resistance25, a bypass capacitance 28 of low reactance being connected between thegrounded cathode of discharge device Island the lower terminal ofresistance 25 removed from the main anode 2d. Potential variations onelectrodes 2| and 22 due to an alternating voltage applied between maincontrol grid l5 and'icathode Of discharge device Ii! are impressed onthe second control grid l8 through a coupling capacitance 29 connectedbetween electrodes 2| and 22 and grid l8.

Therefore, alternating voltages applied directly to control grid I5 andindirectly to control grid 18 cause alternating output signals to appearacross resistance 25, which output signals are then applied-to the gridcircuit of another linearly amplifying discharge device l4. Couplingcapacitance 3| and input resistance 32 are connected in series. and theseries circuit formed thereby is connected in parallel circuitrelationship to the series circuit formed by output coupling resistance25 and low reactance bypass capacitance 28. Capacitance 3! is ofrelatively low reactance and serves essentially, as .a means forblocking-the flow of continuous current from source 23 toresistance 32.The alternating voltage developed across resistance 32; throughcondenser M is applied between the first or control grid and cathode ofdischarge device 14 so as to control the space current therein, whichcurrent normally flows due to the fact that voltage source 23 isconnected between the plate and cathode of discharge device l4 throughthe primary winding 34 of an output transformer 35.

Discharge device I4 is preferably of the pentode type having itssuppressor grid connected to the cathode and with the voltage source 23connected between its screen grid and cathode through a voltage droppingresistance 36. The screen grid is maintained at constant potential inthe presence of signals of frequency corresponding to audio frequenciesby means of low reactance bypass capacitance 31, which is connectedbetween the screen grid and grounded cathode of device l4.

Alternating voltages developed across input resistance 32 are amplifiedlinearly by discharge device M, and appear across the secondary winding39 of transformer 35 which is connected to impress those amplifiedvoltages on the sound reproducing device l2 which is connected acrossthe terminals of secondary winding 39.

The filamentary cathodes of discharge devices H1 and M are preferablyheated by current flowing therethrough as shown in Fig. 1. In such casethe cathodes of discharge devices l0 and I4 are connected in parallelcircuit relationship to voltage source 40.

The circuit thus far described is especially useful as a hearing aidcircuit and, because of the high gain obtained by the use of dischargedevice |0,; only two discharge devices requiring small space current;are necessary for good performance, even though a tone control describedin later paragraphs is provided. Because of the high gain obtained fromthe use of discharge device In, a highly efiicient and useful hearingaid circuit is provided which requires a small current drain fromvoltage source 23. This is particularly true when, as in this instance,resistances 24, 25- and 26 are relatively large.

The volume of signals reproduced by speaker l2 may be controlled byadjusting volume control resistance 24. An adjustment of resistanceZlcauses 'no substantial nonlinearity between the intensity of inputsignals applied to grid 15 and the intensity of signals developed acrossresistance 25 over a large range of input signals. This volume controlcircuit and features thereof are described and claimed in the co-pendingapplication of Gilbert E. Gustafson, Serial No. 518,071 filed January13,1944, and assigned tothe-same assignee as the present application. r

The. tone of signals reproduced on sound reproducing device I2 may becontrolled by connecting resistance [1 and capacitance 38 in the hearingaid circuit thus far described. In general resistance l!- which may beconnected in parallel circuit relationship to piezo-electric microphoneIt servesto reduce the intensity of the low'frequency signal components,and capacitance 30 which may be connected between the electrode 2| andcathode of discharge device I6 serves to reduce the intensity of highfrequency signal components. Different types of tone may be producedcorresponding to thefour positions of the tone control member l3 shownin Figs. 1-4. That is,-tone control member I3 is a short circuiting;member of suchshape that in its clockwise movementiti assumes. positionswhereby; (1-) as in Fig. lithe: capacitance alone is connected in thehearing aid circuit and high frequencies only are suppressed, and (2) asin Fig. 2 when both resistance l1 and capacitance 30 are connected inthe hearing aid circuit both some of the high and some of the lowfrequencies are suppressed, and (3) as in Fig. 3 when resistance I!alone is connected in the hearing aid circuit only some of tho lowfrequencies are suppressed, and (4) as in Fig. 4 when neither theresistance I! nor the capacitance 30 is connected in the hearing aidcircuit neither the low notes nor the high notes are affected.

As mentioned previously, discharge device l operates as two amplifyingstages combined in the envelope of one discharge device with regeneration between the two amplifying stages. The first stage may beconsidered to comprise a triode section including the cathode ofdischarge device IO, main control grid l5 and electrode 2|, whichelectrode operates as the anode of a conventional triode. The secondamplifying stage operates as a pentode and includes control electrodel8, screen electrode 22, suppressor electrode I9 and anode 20.

Measurements substantiate the theory that, in effect, there are twoamplifying stages in the envelope of discharge device l0. That is, thegain measured between cathode and electrode 2| was found to be of theorder of Ell with the device adjusted for good operation, and the gainmeasured from electrode 2| to anode 2i! was of such magnitude that, whenmultiplied by the gain measured between the cathode and electrode 2|, anoverall gain was determined corresponding to the measured overallamplification of the discharge device ID.

The overall gain of discharge device I6 is preferably adjusted byadjusting the amount of regeneration in discharge device I0. It isdesirable to make the amount of regeneration such that linear operationof device I0 is assured consistent with as high gain as possible. Forpurposes of analysis, the two stages may be considered to be equivalentto two separate discharge devices, a triode and a pentode, connected incascade, wherein the regeneration may be symbolized by a negativeresistance of the dynatron type connected across the anode load of thetriode.

The regeneration which comes into being is believed to be due to thenegative transconductance from the second control grid l8 to the screenelectrodes 2| and 22. This negative transconductance effect is producedin accordance with the following considerations: The total cathode spacecurrent of device In is substantially independent of the voltage appliedto the second control grid l8, which controls the distribution ofcurrent between electrode 2| on one side and electrode 22 and anode 20on the other side. Since the two electrodes 2| and 22 are connectedtogether, substantially equal and opposite effects are produced on theelectrodes 2| and 22 and on anode 20 by a change of the voltage on thesecond control grid l8. That is, if electrode I8 is made more negative,more space current tends to flow to electrode 2| and less to electrode22 and anode 2|]. But the decrease in current to electrode 22, whichshares its current with anode 20, is less than the increase in currentto electrode 2 I. Therefore, the net current to electrodes 2| and 22increases with negative potential in electrode l8, and current to anode20 decreases. Consequently, the transconductance between second controlgrid l8 and anode 20 is positive, and the transconductance betweensecond control grid 8 and connected electrodes 2| and 22 is negative.When, as in this instance, the current flowing to connected electrodes2| and 22 produces a voltage drop across external resistance 26 inresponse to variations impressed on the main control grid l5, at least aportion of that voltage drop is fed back to the second control grid l8,through coupling capacitance 29 for further controlling the electronstream in discharge device In regeneratively by voltage on electrode I8.That is, in Fig. 1, it appears as though a negative resistance effect,which causes regeneration for signals of audio frequencies, occursbetween connected electrodes 2| and 22, second control grid I8 and thecathode of device l0, considered as a group, since coupling condenser 29is of low reactance for signals of audio-frequency.

In the particular amplifier circuit shown in Fig. 1, when properlyadjusted, the increase in triode gain in discharge device It! due toregeneration is about 70%. It is possible, however, to increase theregeneration to such an extent that a gain of about 2,000 is obtained inthe triode section alone. This condition is not satisfactorily stable.

In general, any means that causes a change in transconductance betweenthe control electrode l8 and anode 20 of device Ill may be used tocontrol the negative transconductance between control grid 8 andconnected electrodes 2| and 22 so that the amount of regeneration indischarge device I0 is also changed. That is, when the positivetransconductance between grid l8 and the anode 20 is high, the negativetransconductance between connected control grid l8 and grids 2| and 22is also high.

For that reason, the amount of regeneration is adjusted by adjusting theanode output resistance 25. When anode resistance 25 is decreased belowa critical resistance, the transconductance between the control grid I8and anode 2|) is increased so that device lll breaks into oscillation,due to the corresponding increase in negative transconductance betweencontrol grid l8 and connected grids 2| and 22. The signal voltagedeveloped across resistance 25 is, of course, equal to the signalcurrent flowing therethrough multiplied by the resistance of resistance25. By making resistance 25 high the transconductance of the device islowered but the voltage developed across resistance 25 tends to remainconstant.

It is desirable to make the resistance 25 as large as possible,consistent with high gain, not only for suppressing oscillation indevice II] but also for decreasing the current drain from voltage source23. Also, resistance 25 is made large so as to assure linearamplification of high intensity signals applied between the grid 5 andcathode of device Iii. Since a reduction of anode to cathode conductancetakes place when the anode potential is reduced low enough to causeoperation along the curved portion of the anode voltage anode currentcharacteristic in some distion between the triode and pentode sectionsof discharge device ii] is to adjust the continuous operating potentialof connected grids 2| and 22, for example, by adjusting the resistance25. In general, the smaller the resistance 26, the more discharge devicein approaches a condition of self-oscillation. When resistance .26 ismade :large, 'spajce current in electrodes 2| and 22 is reduced.

=Regenerationin discharge device it is also affected by the amount ofelectron emission from the filament or cathode. It isnot uncommon for toprolonged current draintherefrom, the overall amplification of device IDtends to increase and thus a compensation is provided for the loss ofamplification resulting from the decrease in voltage of source 23 inprolonged operation of the hearing aid. Under certain conditions achange in voltage of the heating source 150 may cause a condition ofself-oscillation. Variation invthe circuit behavior due to change inheating source voltage 30 is minimized greatly When resistances 25 and2.6 are suitably large,

.A possible explanation for the increase in regeneration when filamentemission is decreased may perhaps be that there is a reductioninemission so that all electrode. potentials rise. Since large anode andscreen electrode resistances are used to force operation of the pentodepart of the discharge device on a steeply sloping part of the anodevoltage and anode current characteristic, when anode and screenpotentials rise, the dynamic transconductances of both anode and screenelectrode increase with a consequent. increase in gain and regeneration.The negative transconductance between the grid I15 and anode 20 isincreased and correspondingly the regeneration in the triode stage isincreased. When resistances Z and 26 are properly adjusted for high gainconsistent with low current consumption in accordance with principlesdiscussed herein, reduction of voltage of source 30 has appreciably noeffect on operation of device It over a large range of change of voltageof source til.

Under certain conditions of adjustment, the resistances 24 and 25 mayhave such relative magnitudes that. the electrons constituting the spacecharge are attracted to connected electrodes 2i and 22 and anode 2c insuch proportion that the respective transconductances are substantiallyunaltered as the space charge is reduced when the filament emission isdecreased. This has been observed when resistance 25 is of the order of709,000 ohms and resistance 25 is of the order of 80,000 ohms, device Itbeing a 1R5.

In ademonstration to show the maximum overall gain obtainable by usingdevice Iil, the plate load resistance .25 was reduced so that it wasalmost small enough for the production of self-sustained oscillationsin'discharge device H3. With 100 microvolts input, the output voltagewas 1.4 volts. This corresponds to an overall g ain-of lei-n00 in-onedischarge device. Such gain is usually not conveniently usable becauseof the greater difhculty ofproportioning resistances 25 and 23 to avoidself-sustained oscillationsas the voltage of source 46 drops.

The circuit-shown inFig. 1 has much-advantageover conventionalregeneration circuits. In general, in order to produceregeneration-insultable manner in an electrical circuit, it is necessarythat an output signal of an amplifier be transferred back to the inputcircuit of the ainplifier through circuits which shift the phase of thesignal by a total amount approaching 360 or a multiple thereof includingthe phase shift in the amplifier. This necessitates either carryingthephase shifting operation over two tubes, one'or which shifts phase orusing a transformer for phase inversion. Systems incorporating suchmeans are inferior in stability and frequency range to the amplifyingsystem as disclosed herein incorporating a single discharge device witha resistance load, because such systerns must all transfer signalsthrough at least two filter meshes in the feedback loop to obtain the360 phase shift through the entire feedback loop, while the presentarrangement requires but one 'm'esh including resistances 2-6 and 27 andcondenser 29 to complete the entire feedback loop for shifting signals360; 01', if it be preferred, 0. That is, there is no phase shiftdesired in translation of signals through the single mesh comprisingresistances "26 and 27 and condenser 29, signals on grids 2i and 22being effective to produce regeneration when impressed in phase on gridIll. The regenerative effect is correspondingly constant over afrequency band limited only by the signal transferring ability for whichthe mesh including resistances 2:5 and 27 and condenser 29 is adjusted.

When a signal of increasing instantaneous intensity is applied betweenmain control grid l5 and the cathode of discharge device ill, oninstantaneously decreasing current correspondingly flows throughresistance 25. That is, the effective transconductance between grid 15and anode 28 is negative in character. This is in line with the theorythat device [0 includes two amplifying devices in cascade wherein, as iswell known, the effective transconductance between the grid of the firstdevice and the anode of the second device is negative in character.

'One of the important features of the present invention resides in thefact that the instantaneous intensity of current flowing throughresistance 25 varies substantially linearly with the instantaneousintensity of a signal applied between main control grid I5 and cathodeof discharge device lil. In other words, the effective transconductancebetween control grid 15 and anode 20 is substantially constant over therange of intensity of signals applied between grid [5 and its cathode.

When the resistance 25 is increased, the amount of regeneration indischarge device I!) is decreased, the amount of space current flowingto anode 28 is reduced, and consequently the overall gain i decreased.Also, when the resistance 26 is increased, the amount of space currentflowing to screen-e21 and 22 isreduced, and the overall gain isdecreased. When resistance 25 or resistance 26 is made small the spacecurrent flowing through the corresponding resistances 25 or 26 isincreased and, the overall amplification increases to a point where theamplifying circuit changes abruptly intoa state of self-oscillation.Just before such self-oscillation begins, amplification is great, thereis enhanced linear or frequency distortion, enhanced instability and theanode voltage is not sufficient to accommodate signal voltage changes,so that signals so greatly amplified are distorted. Such adjustment isundesirable. By making the resistance 25 very large. as taught for highgain and low current consumption, regeneration is reduced sufficientlythat the device It is highly linear and yet has great gain foramplification of small signals. It is thus seen that, in. general, whenthe amount of space current flowing in device H) is reduced, the amountof gain is reduced, and conversely, when the gain is increased, theamount of space current flowing in device in is increased also. It isunderstood, of course, that such space current flow in varying degree isthe same as varying current drain from source 23.

Adjustment made in increasing the size of resistance 25 to cause morelinear operation and to suppress self-oscillation and reduce dischargecurrent is quite different from that present in known self-oscillationcircuits having a resistance in the anode circuit. That is, in theamplifyingoscillating circuit of a multivibrator, having a resistance inthe anode circuit, self-oscillation is suppressed when such resistanceis made smaller, whereas in the arrangement shown in- Fig. 1selfoscillation is suppressed by increasing resistance 25.

With a constant output voltage across resistance 25, the relationshipbetween intensity of input'voltage applied to main control grid 15 andthe intensity of such constant output voltage becomes more linear as theresistance 25 is increased. When resistance 25 is so increased, asmaller amount of output signal current through resistance 25 isnecessary to produce such constant voltage output, and regeneration indevice I is suppressed suitably, thus affording more linear operatingconditions. Consequently, when plate coupling resistance 25 isincreased, compensation is introduced for nonlinear conditionsprevailing when resistance 25 is small.

Since the resistances 25 and 26 influence regeneration and linearoperating conditions of device IO, and since they are connected inseparate series circuits, including the common voltage source 23, theirrelative sizes influence the operating conditions of device l0.

When resistance 25 is increased, the overall transconductance of devicel0 tends to decrease; and it is then desirable to have resistance 26relatively small, consistent with linear operating conditions, so as toprovide a compensation for the loss in gain due to increase inresistance 25.. That is, when the ratio of resistance 25 to resistance26 is increased, the overall gain of device In tends to be less affectedby a change in resistance 25 even though a greater and more linearvoltage amplification is produced when resistance 25 is increased. Thatis, when resistance 26 is small compared to resistance 25, greaterlinearity is achieved than when the resistance 25 is comparable toresistance 26. Also, when resistance 26 is small compared to resistance25; variations in heating voltage source 46 have little effect on theoperating conditions of discharge device III.

In a practical embodiment of the present invention, the various circuitelements described herein had the following properties: discharge deviceI 6-1R5 R. C. A. type, resistance l6-5 megohms, resistance I'I470,000ohms, resistance 24400,000 ohms, resistance 25--680,000 ohms, resistance2668,000 to 82,000 ohms, resistance 21-10 megohms, resistance 32-4.?megohms, condenser 28-.1 microfarad, condenser 29100 micromicrofarads,condenser 30-.02 microfarad, condenser 3l-.001 microfarad.

While I have shown and described the particular embodiments of myinvention, it will be obvious to those skilled in the art that changesand modifications may be made without departing from my invention in itsbroader aspects, and I, therefore, aim in the appended claims to cover10 7 all such changes and modifications as fall within the true spiritand scope of my invention.

I claim:

1. An amplifier circuit including an electron discharge device having ananode, cathode, first control electrode, second control electrode and atleast one grid electrode, said grid electrode being interposed betweensaid first and second control electrodes, a source of anode operatingpotential for said electron discharge device, said source havingpositive and negative terminals, a capacitor connected between saidsecond control electrode and said grid electrode to couple voltagechanges therebetween, a first resistance connected between said gridelectrode and said positive terminal of said source, second and thirdresistances connected between said first and second control electrodes,respectively, and said cathode electrode to properly load said controlelectrodes, and an anode load resistance connected between said anodeelectrode and said positive terminal of said source, said electrondischarge device having a normal range of anode load resistances, saidamplifier circuit tending to oscillate when said anode load resistancelies in said range, said anode load resistance being chosen to have amagnitude greatly in excess of the magnitudes in said normal range ofanode load resistances, whereby said amplifier is prevented fromoscillating.

2. An amplifier circuit including an electron discharge device having ananode, cathode, first control electrode, second control electrode and atleast one grid electrode, said grid electrode being interposed betweensaid first and second control electrodes, a source of anode operatingpotential for said electron discharge device, said source havingpositive and negative terminals, a coupling network connected betweensaid second control electrode and said grid electrode to couple voltagechanges therebetween, a first resistance connected between said gridelectrode and. said positive terminal of said source, second and. thirdresistances connected between said first and second control electrodes,respectively, and said cathode electrode to properly load said controlelectrodes, and an anode load resistance connected between said anodeelectrode and said positive terminal of said source, said electrondischarge device having a normal range of anode load resistances, saidamplifier circuit tending to oscillate when said anode load resistancelies in said range, said anode load resistance being chosen to have amagnitude greatly in excess of the magnitudes in said normal range ofanode load resistances, whereby said amplifier is prevented fromoscillating.

JOHN G. PRENTISS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,228,080 Herold Jan. 7, 19412,342,492 Rankin et al Feb. 22, 1944 2,226,561 Herold Dec. 31, 19402,287,280 Terman June 23, 1942 2,235,817 Freeman Mar. 25, 1941 2,262,916Boucke 2 Nov. 18, 1941 2,214,614 Hunt Sept. 10, 1940

